Objective lens for optical pickup device, optical pickup device and optical information recording/reproducing apparatus

Abstract

An objective lens used for an optical pickup device wherein the optical pickup device includes: light source; and a converging optical system including the objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, and the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, the objective lens being a plastic single lens and satisfying following formulas when NA is an image-side numerical aperture required for recording and/or reproducing information to the optical information recording medium and f (mm) is a focal length of the objective lens. Even in a plastic single lens having a high NA, thermal aberration does not increase excessively and in a plastic single lens of a refraction type, thermal aberration within the temperature range of practical use in an optical pickup device is suppressed within an allowable range. NA≧0.8  (1) 1.0>f>0.2  (2)

1. An objective lens used for an optical pickup device, wherein the optical pickup device comprises:
 a light source; and 
 a converging optical system including the objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, 
 wherein the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, 
 wherein the objective lens is a plastic single lens that comprises a ring-shaped phase structure on at least one optical surface, the ring-shaped phase structure comprising a plurality of ring surfaces and formed so that adjacent ring surfaces generate a predetermined optical path difference for incident light, 
 wherein when a ring surface including an optical axis is called a central ring surface, a ring surface adjacent to an outside of the central ring surface is formed to be displaced in the optical axis direction so as to have a shorter optical path length than the central ring surface, a ring surface at a maximum effective diameter position is formed to be displaced in the optical axis direction so as to have a longer optical path length than a ring surface adjacent to an inside thereof, 
 wherein the objective lens satisfies following formulas:
   NA≧0.8
 
   500≧λ 0 >350
 
   0.40<( X 1 −X 2)·( N− 1)/(NA· f ·√(1+| m |))≦0.63
 
 
 where NA is an image-side numerical aperture of the objective lens, which is required for recording and/or reproducing information for the optical information recording medium, f (mm) is a focal length of the objective lens, λ 0  (nm) is a design wavelength of the objective lens, X1 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on a light source side and an optical surface on the light source side in a most peripheral portion of an effective diameter (position of the NA on a surface on the light source side to which a marginal light beam is incident), X2 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on an optical information recording medium side and an optical surface on the optical information recording medium side in a most peripheral portion of an effective diameter (position of the NA on a surface on the optical information recording medium side to which a marginal light beam is incident), N is a refractive index of the objective lens at the design wavelength of λ 0 , and m is an image formation magnification of the objective lens, 
 wherein X1 is a plus in a case of measuring X1 in a direction of the optical information recording medium with reference to the tangent plane, and minus in a case of measuring X1 in a direction of the light source, and 
 wherein X2 is plus in a case of measuring X2 in a direction of the optical information recording medium with reference to the tangent plane and minus in a case of measuring X2 in a direction of the light source. 
 
     
     
       2. The objective lens for the optical pickup device of  claim 1 , wherein the objective lens satisfies the following formula:
   0.8 <d/f< 1.8 
 where d (mm) is a lens thickness in an optical axis of the objective lens. 
 
     
     
       3. The objective lens for the optical pickup device of  claim 1 , wherein the objective lens has a spherical aberration characteristic that spherical aberration changes in an undercorrected direction when a wavelength of the incident light changes to a longer wavelength. 
     
     
       4. The objective lens for the optical pickup device of  claim 1 , wherein a total of ring surfaces is from 3 to 20. 
     
     
       5. The objective lens for the optical pickup device of  claim 1 , wherein when Δ j  (μm) is a step amount of an arbitrary step of steps in the optical axis direction at a boundary of mutually adjacent ring surfaces in a ring-shaped phase structure formed in a region from a height of 75% to a height of 100% of an effective diameter maximum height of the optical surface on which the ring-shaped phase structure is formed and n is a refractive index of the objective lens at a design wavelength of λ 0  (nm), mj represented by
     mj =INT( X ) 
 (wherein X=Δ j ·(n−1)/(λ 0 ·10 −3 e) and INT (X) is an integer obtained by half adjust of X) is an integer not less than 2. 
 
     
     
       6. An optical pickup device, comprising:
 a light source; and 
 a converging optical system including an objective lens for converging a light beam emitted from the light source to an information recording surface of an optical information recording medium, 
 wherein the optical pickup device is capable of recording and/or reproducing information by converging the light beam emitted from the light source to the information recording surface of the optical information recording medium with the converging optical system, 
 wherein the objective lens is a plastic single lens that comprises a ring-shaped phase structure on at least one optical surface, the ring-shaped phase structure comprising a plurality of ring surfaces and formed so that adjacent ring surfaces generate a predetermined optical path difference for incident light, 
 wherein when a ring surface including an optical axis is called a central ring surface, a ring surface adjacent to an outside of the central ring surface is formed to be displaced in the optical axis direction so as to have a shorter optical path length than the central ring surface, a ring surface at a maximum effective diameter position is formed to be displaced in the optical axis direction so as to have a longer optical path length than a ring surface adjacent to an inside thereof, 
 wherein the objective lens satisfies following formulas:
   NA≧0.8
 
   500≧λ 0 >350
 
   0.40<( X 1− X 2)·( N− 1)/(NA· f ·√(1+∥ m |))≦0.63
 
 
 where NA is an image-side numerical aperture of the objective lens, which is required for recording and/or reproducing information for the optical information recording medium, f (mm) is a focal length of the objective lens, λ 0  (nm) is a design wavelength of the objective lens, X1 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on a light source side and an optical surface on the light source side in a most peripheral portion of an effective diameter (position of the NA on a surface on the light source side to which a marginal light beam is incident), X2 (mm) is a distance in an optical axis direction between a plane that is perpendicular to an optical axis and tangent to a top of an optical surface on an optical information recording medium side and an optical surface on the optical information recording medium side in a most peripheral portion of an effective diameter (position of the NA on a surface on the optical information recording medium side to which a marginal light beam is incident), N is a refractive index of the objective lens at the design wavelength of λ 0 , and m is an image formation magnification of the objective lens, 
 wherein X1 is a plus in a case of measuring X1 in a direction of the optical information recording medium with reference to the tangent plane, and minus in a case of measuring X1 in a direction of the light source, and 
 wherein X2 is plus in a case of measuring X2 in a direction of the optical information recording medium with reference to the tangent plane and minus in a case of measuring X2 in a direction of the light source. 
 
     
     
       7. The optical pickup device of  claim 6 , wherein the objective lens satisfies the following formula:
   0.8< d/f< 1.8 
 where d (mm) is a lens thickness in an optical axis of the objective lens. 
 
     
     
       8. The optical pickup device of  claim 6 , wherein the objective lens has a spherical aberration characteristic that spherical aberration changes in an undercorrected direction when a wavelength of the incident light changes to a longer wavelength. 
     
     
       9. The optical pickup device of  claim 6 , wherein a total of ring surfaces is from 3 to 20. 
     
     
       10. The optical pickup device of  claim 6 , wherein when Δ j  (μm) is a step amount of an arbitrary step of steps in the optical axis direction at a boundary of mutually adjacent ring surfaces in a ring-shaped phase structure formed in a region from a height of 75% to a height of 100% of an effective diameter maximum height of the optical surface on which the ring-shaped phase structure is formed and n is a refractive index of the objective lens at a design wavelength of λ 0  (nm), mj represented by
     mj =INT( X ) 
 (wherein X=Δ j ·(n−1)/(λ 0 ·10 −3 ) and INT (X) is an integer obtained by half adjust of X) is an integer not less than 2.